In a wide variety of situations, environmental noise presents substantial problems ranging from stress, safety hazards and annoyance to physiological damage. Until recently, efforts at noise control have centered primarily about either reducing the amount of noise created or using sound absorbing materials to absorb and deaden environmental noise.
For example, in the case of an automobile, engine noise is first suppressed through the use of a muffler. It then remains to prevent residual engine noise, road noise, wind noise and the like from entering the passenger compartment of the vehicle. Typically, this is done by lining the cabin area of the vehicle with sound absorbing and sound shielding material. Such materials are applied to the floor, the ceiling and sidewalls of the cabin area.
These materials generally perform two functions, namely, the function of shielding the passenger cabin from noise outside the vehicle and absorbing any noises which enter the passenger cabin.
While this sort of approach does achieve a large degree of success in protecting the passenger from environmental noise, still a substantial amount of noise remains.
In other areas of technology, conventional noise reduction techniques are of little or no value. For example, in the case of a dentist's chair, the above techniques provide little or no relief from the stress inducing noises produced by the dentist's drill. While to some extent a portion of the noise created by the dentist's drill is transmitted through the tissues in the head of the patient to the ear, this represents a relatively minor part of the problem. Nevertheless, because of the difficulties involved in approaching the remaining air propagated repetitive noise, virtually nothing has been done in addressing the problem of the remaining noise.
Turning to another area in the health care field, the extremely high levels of noise experienced by the patient during the operation of a nuclear magnetic resonance imaging device is of an extremely high level. This, coupled with the possibly very serious nature of the illnesses involved, combine to create an extremely high level of physical and psychological discomfort. In addition, because of the nature of this equipment, including both its physical configuration and electronic characteristics, conventional noise reducing approaches cannot be employed.
In the past, dynamic noise cancellation systems have been devised. Such systems generally involve the generation of a second audio signal which is equal in magnitude to the noise to be eliminated but opposite in sign at the point where one desires to achieve noise cancellation.
Typically, a microphone is positioned at the point where cancellation is desired. The microphone generates a signal which is indicative of the amplitude of noise at that point, and this signal is sent to a processor which generates the cancellation signal and sends it to an actuator which is often a conventional loudspeaker, which in turn produces a cancellation audio signal at the point where quiet is desired.
While this sort of approach certainly does go a long way in reducing environmental noise problems, such systems have some limitations when it comes to achieving noise cancellation over a desired range of areas.
A possibly major problem in dynamic noise cancellation systems is the positioning of a microphone near the point where one wishes to achieve effective dynamic noise cancellation. This results in a number of obvious problems, namely, the introduction of microphones close to, for example, the ears of an individual in a car or other vehicle. This introduces potential safety hazards, as well as inconvenience and potential discomfort.
In addition, such approaches present aesthetic problems which, for example, in the context of a passenger automobile, are an important negative consideration to potential buyers. More importantly, protruding microphones and the like, besides presenting a hazard to the safe operation of the vehicle, as noted above, also, in the event of an accident may do damage to the eyes of an individual in the automobile.